The present invention relates to a self-baking electrode for use in low electric reduction furnaces, and in particular it refers to a container for forming self-baking electrodes to be used in low electric reduction furnaces. The invention also relates to a method of forming a self-baking electrode using this container as well as the electrode formed thereby. Finally, the invention relates to the use of a self-baking electrode formed in this container for manufacturing silicon alloys.
Conventional self-baking electrodes are formed in a segmented cylindrical container (sections of casing) arranged vertically extending from the inside of the furnace stack until the uppermost height of the building thereof. The upper end of the cylindrical container is open in order to allow the addition of unbaked electrode paste, which when submitted to heating, due to the heat added in the area of supply of electric operating current to the electrode, softens, melts, discharges volatile products, and is thereafter baked into a solid carbon electrode. As the electrode is consumed in the furnace, the electrode is lowered and new sections of casing are installed at the top of the column, where the unbaked electrode paste is then added.
A conventional electrode of this type is equipped with metallic ribs attached to the inner surface of the vertical casing, the ribs extending radially relative to the axis of the electrode. When a section of casing is installed at the top of the electrode column, its casing and its ribs are welded to the casing and the ribs of the already installed segment in order to obtain continuity of the ribs in the vertical direction. The ribs serve to support, conduct electric current, and heat into the electrode during the baking process. To compensate for the consumption of the electrode, the same is lowered into the furnace by means of the sliding mechanism.
When conventional electrodes of this type are used, the electrode container casing and the inner ribs melt when the electrode is being consumed in the furnace. The metal content of the casing and the ribs is transferred to the product in the furnace. Since the container casing and the inner ribs usually are made from carbon steel, such self-baking electrodes can not be used in electric reduction furnaces for the production of high-grade silicon alloys, as the iron content in the produced material will become unacceptable.
In the 1920""s it was proposed to conduct heat into the self-baking electrodes through inserts of pre-baked carbon bodies in the unbaked electrode paste. In Norwegian patent NO 45408 there is disclosed a method for the production of self-baking electrodes wherein pre-baked carbon bodies are placed in the periphery of the electrodes and are kept in place by the unbaked electrode paste. The carbon inserts are not attached to the casing, but are merely kept in place by the unbaked electrode paste, and when the electrode is baked, by the baked electrode paste. In order to keep the carbon inserts in place before, during, and after the baking of the electrode paste, it is necessary that each casing be fully filled with hot liquid electrode paste when a new length of casing is installed at the top of the electrode column, since it is only the electrode paste that keeps the carbon inserts in place against the inner wall of the casing, which may render difficult the calcination of the central part of the electrode. Those carbon inserts will not function in the same manner as the ribs used in the conventional self-baking electrodes. The method in accordance with Norwegian patent NO 45408 has for these reasons not found any practical use.
There have been proposed over the years, however, a number of modifications of the conventional self-baking electrodes not having inner ribs made of steel in order to avoid contamination of the silicon produced in the furnace caused by the iron product of the casing and the ribs.
In Norwegian patent NO 149451 there is disclosed a self-baking electrode wherein the electrode paste contained in a casing devoid of ribs, is being baked above the location where the electric operating current is supplied, and wherein the casing is removed after baking, but before having been lowered down to the place where the electric operating current is supplied. An electrode is produced in this manner which has neither casing nor ribs.
That kind of electrode has been used in low furnaces for the production of silicon, but nevertheless having the disadvantage when compared with conventional pre-baked electrodes in that costly equipment must be installed in order to bake the electrode and to remove the casing from the electrode.
In U.S. Pat. No. 4,692,929 there is described a self-baking electrode to be used with electric furnaces for the production of silicon. The electrode comprises a permanent metal casing without ribs and a support frame for the electrode comprising carbon fibers, wherein the electrode paste is baked upon the support frame and wherein the baked electrode is being held by the support frame. That electrode has the disadvantage that special fastening equipment must be arranged above the top of the electrode in order to hold the same using the support structure comprising carbon fibers. Furthermore, it may be difficult to have the electrode slide downwards through the permanent casing when the electrode is being consumed.
In U.S. Pat. No. 4,575,856 there is disclosed a self-baking electrode having a permanent casing without ribs, wherein the electrode paste is being baked over a central graphite core and wherein the electrode is being held by the graphite core. That electrode has the same disadvantages as the electrode according to U.S. Pat. No. 4,692,929 and in addition the graphite core is prone to breakage when the electrode is subjected to radial forces.
The methods cited above for the production of a self-baking electrode without ribs suffer from the disadvantage that they can not be used for electrodes with a diameter above 1.2 m without substantially increasing the probability of breakage. However, conventional self-baking electrodes are used that have diameters of up to 2.0 m.
U.S. Pat. No. 5,778,021 discloses a container for the formation of self-backing electrodes for use in low electric reduction furnaces, the container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs perpendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing.
The present invention relates to a self-baking electrode for use in low electric reduction furnaces, and refers particularly to a container for the formation of self-baking electrodes to be used in low electric reduction furnaces, allowing the manufacture of silicon alloys with iron content as low as 0.35%, the container comprising a cylindrical casing split in two parts containing therein a plurality of ribs uniformly attached perpendicularly along the inner surface of the casing lengthwise along the cylindrical casing wherein the cylindrical casing and ribs are made of stainless steel plates.
It is therefore an object of the present invention to provide a container for the formation of self-baking electrodes to be used in low electric reduction furnaces, comprising a cylindrical casing containing in the inside thereof a plurality of ribs perpendicularly attached along the inner surface of the casing in the longitudinal direction of the cylindrical casing wherein the cylindrical casing is made of stainless steel plates and the ribs are made of stainless steel plates. If desired, the container can be split in 2 parts.
It is another object of the invention to provide a method of forming a self baking electrode comprising adding unbaked electrode paste to an electrode container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs perpendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing and heating the paste by a method selected from heat supplied by a heater, heat generated by the introduction of electric energy, and a combination thereof.
It is another object of the invention to provide an electrode produced by the above process.
It is yet another object of the invention to provide a method for manufacturing silicon alloys with low iron content using a self baking electrode, the improvement comprising forming the self baking electrode in an electrode container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs perpendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing.